An information recording medium on which information can be only once recorded by means of a laser beam is known as a recordable compact disc (CD-R), and has been getting widely used because it can be produced on a relatively small scale and at a low cost, as compared with a conventional compact disc (CD).
The optical disc of CD-R type generally has a multi-layer structure typically comprising a disc-shaped transparent substrate (support), a recording layer comprising an organic dye, a light-reflecting layer made of a metal such as gold, and a protective layer made of a resin overlaid in order. The information can be recorded by irradiating the disc with a laser beam of a near infrared region (usually around 780 nm). By the irradiation with the laser beam, the irradiated area of recording layer is locally heated to change the physical or chemical characteristics, and thus pits are formed in the recording layer. Since the optical characteristics of the formed pits differ from those of the area having been not irradiated, the information is recorded. The recorded information can be read by the reproducing procedure comprising the steps of: irradiating the recording layer with the laser beam having the same wavelength as that of the laser beam employed in the recording procedure, and detecting the light-reflection difference between the pits and the other area.
The above CD-R having a recording dye layer is desired to keep a high light-resistance after recording, as well as high information reproductivity and such a high light-reflection that the information can be reproduced by means of a commercially available CD player. As described above, CD-R has been getting widely used in these days, and accordingly it is often used under severe conditions (such as extreme fluctuation of ambient temperature and/or humidity). Therefore, the light-resistance of CD-R is now very important subject.
As a dye compound for the recording layer, a cyanine dye having a benzoindolenine structure (in which a benzene ring is fused with the indolenine skeleton) is advantageously used, as is described in Japanese Patent Provisional Publications No. 64-40382 and No. 64-40387. In combination with the above-mentioned cyanine dye, various singlet oxygen quenchers are generally employed as an anti-fading agent so as to improve the light-resistance of the recording dye layer. For example, nitroso compounds (Japanese Patent Provisional Publication No. H2-300288) and nickel complexes (Japanese Patent Provisional Publication No. H4-146189) are well-known singlet oxygen quenchers. Besides them, electron accepting compounds such as tetracyanoquinodimethane (TCNQ) are proposed to effectively enhance the light-resistance of the recording layer containing the above cyanine dye (Japanese Patent Provisional Publication No. 63-64794).
According to studies by the inventors of the present invention, however, the above singlet oxygen quenchers and the electron accepting compounds may not give a satisfactory light-resistance to the recording layer containing the cyanine dye.
In Japanese Patent Provisional Publication No. H4-175188, a cyanine dye preferably employed for CD-R is proposed. According to the publication, the proposed dye has a light-reflection higher than that of the dye described in Japanese Patent Provisional Publication No. 64-40382, and also it exhibits a high light-resistance in the presence of the conventional anti-fading agents. However, although the publication says that the dye can have an anion of 1 to 3 valence, concrete examples of the dye having only a monovalent anion are disclosed, and there is no description about those of the dye having a di- or tri-valent anion.
It is an object of the present invention to provide an optical information recording medium having an improved light-resistance after recording. Particularly, it is an object of the invention to provide an optical information recording medium which keeps a high light-resistance during its storing under severe conditions such as a high temperature, a high humidity and an extreme fluctuation of temperature and/or humidity.